Pathlength controller for ring laser gyroscope

ABSTRACT

The present invention provides for initializing the particular intensity peak selected to operate about, based upon the initial temperature of the gyroscope in order to maintain a selected integer number of wavelengths as the initial cavity path length from start up to start up. In addition, the present invention provides an optimum available movement of the mirrors due to changing the voltage over the available voltage swing for the anticipated range and direction of temperature changes from the start up temperature, over which the gyroscope is expected to operate.

FIELD OF INVENTION

The present invention relates to ring laser gyroscopes and morespecifically to pathlength control for ring laser gyroscopes whichselects an initial operating mode based upon gyroscope startuptemperature and the expected range of operating temperatures, andcompensates for temperature changes during the operation of thegyroscope which have an effect on the gyroscope path length.

BACKGROUND AND SUMMARY OF THE INVENTION

It is well known in the art that ring lasers may be adapted to be usedas angular rate sensors. Such ring lasers are known as ring lasergyroscopes. In such a ring laser gyroscope the difference betweenfrequencies of counter-propagating radiant energy in the form of laserbeams is a measure of the rate of angular rotation of the structure inwhich the propagating waves are traveling. Further background anddiscussion of the basics of ring laser gyro operation may be found, forexample, in Killpatrick, "The Laser Gyroscope", IEEE Spectrum, October,1967; Coccoli, "An Overview of Laser Gyroscopes", P-684 12th JointServices Data Exchange for Inertial Systems, Norfolk, Va., October,1978; and U.S. Pat. Nos. 4,383,763 to Hutchings et al. assigned to theassignee of the present application and 4,422,762 to Hutchings et al.assigned to the assignee of the present application, the disclosures ofeach of which are hereby incorporated by reference.

It is known in the art, as shown in the '762 Patent to Hutchings notedabove, to dither the mirrors of the ring laser gyro, or some of them, inorder to avoid the phenomenon known as lock-in which occurs when theangular rotation rate of the ring laser gyro is close to zero, and isthe result of the operating characteristics of a ring laser gyro, e.g.,back-scattering of light from the mirrors defining the propagationcavity. In addition, as discussed in that patent, it is known in the artto translate one or more of the mirrors under the control of apiezoelectric transducer, upon which the mirror is mounted,inwardly andoutwardly to adjust the propagation cavity length in order to maintainthe cavity at a selected integer multiple of the wave length of thelaser beam. That patent also discusses a manner of accomplishing this bysensing the AC envelope of the intensity of the counter-propagatingbeaxs summed together, which envelope has a peak at an optimum cavitypath length, when the path length is an integer multiple of the wavelength of the counter-propagating beams.

The piezoelectric transducers used to translate the mirrors to modifythe path length have some finite sweep range determined by such thingsas the range of voltages applied to the piezoelectric transducer, thematerial of the piezoelectric transducer and the physical structure ofthe piezoelectric transducer and mirror assembly. Typically, thepiezoelectric transducer is capable of translating the mirrorssufficently to pass through a plurality of, for example, from four tosix intensity maxima, i.e., to change the cavity path length through arange of about six integer multiples of the beam wave length.

It is also known that temperature variation due to, for example,changing the environment in which the laser gyroscope is operating, willcause thermal expansion or contraction of the laser gyroscope body andthus the cavity containing the path for the counter-propagating beams.This changes the cavity path length.

The present invention provides for initializing the particular intensitypeak selected to operate about, based upon the initial temperature ofthe gyroscope in order to maintain a selected integer number ofwavelengths as the initial cavity path length from start up to start up.In addition, the present invention provides an optimum availablemovement of the mirrors due to changing the voltage over the availablevoltage swing for the anticipated range and direction of temperaturechanges from the start up temperature, over which the gyroscope isexpected to operate. This will be referred to herein as selecting themode for the gyro. It is based upon which of the possible intensitymaxima corresponds to a selected path length, and optimizes the expectedmovement of the path length control mirror or mirrors over the range oftemperature changes which the gyroscope is expected to experience inoperation,based upon the initial temperature. In this manner, the samepath length, (an integer multiple of the laser wavelength), is selectedfor each start-up,regardless of start up temperature. Ring laser gyroscale factor variables vary with the cavity path length. Scale factorsrelate the output beat frequency of the counter-rotating beams to theangular variation of the gyro, i.e., gyro input. Therefore, the presentinvention initiates each operation at a mode which retains the samescale factor variables from start-up to start-up. In this manner, also,the necessity for a mode hop to another peak intensity during theoperation of the gyro may be obviated. Such a mode hop would benecessitated if the changes in the path length resulting fromtemperature changes are extensive enough to be beyond the capability ofthe piezoelectric transducers on the mirror or mirrors used for pathlength control to translate the mirrors sufficiently to maintain thedesired path length.

Recognizing the need for improved cavity path length control, it is ageneral object of the present invention to provide an improved methodand apparatus for maintaining the correct cavity path length in ringlaser gyro, including a method and apparatus for initially selecting anoptimum operating range for expected temperature variations.

A feature of the present invention involves, initially, slewing thecavity length control mirror or mirrors through their entire operationalrange and sampling the beam intensity and storing the position of theintensity peaks. Thereafter, a particular peak is selected about whichto maintain zero detuning, based upon the initial operating temperatureof the gyoroscope.

A further feature of the present invention is to employ a plurality of,for example, three selected operating temperature ranges, for example,one below a temperature T₁, one between a temperature T₁ and atemperature T₂ and one above the temperature T₂. The initial temperatureof the gyroscope in these empirically selected temperature bands at thetime the system is turned on is employed as a means for optimallyselecting a particular peak about which to carry out detuning. Theselection results in the initial cavity path length being a selectedinteger multiple of the laser wavelength and detune being carried out tomaintain that integer multiple in this operating mode. Also, theselection results in selecting an initial value of the voltage output tothe piezoelectric mirror drive transducers which will allow for agreater change in the direction of high or low temperature response, ifthe initial temperature is, respectively, low or high, and about anequal range of change in either direction where the start-up temperatureis initially in a mid-range. The piezoelectric transducer, therefore,will have sufficient operating range to maintain detune about theselected peak from, e.g., -54° C. to +80° C.

Another feature of the present invention is that for some lasergyroscopes,depending upon various parameters of the structure andoperation of the ring laser, may be initially set at the proper modebased solely upon determing the start-up temperature. Where the numberof mode peaks occurring over the entire sweep of the CLC mirrors is,e.g., four or less, and/or where the laser exhibits no significantsecondary peaks, it may be possible to empirically determine a voltageto which to slew the CLC mirrorswhich will insure that by engaging thetracking function from such an initial position of the mirrors, theresult is to attain the desired mode about which detune is controlled.

Of course, it will be understood that a ring laser gyroscope, accordingto the present invention, has the capability to slew the path lengthcontrol mirrors to a position to employ another of the intensity maximaas a detuning point. This might occur, if the temperature change causesa path length variation beyond the capability of the path length controlmirrors to correct in the initial mode selected. However, mode shiftingresults in the injection of inaccurate gyro output data while suchslewing occurs. It will also result in having some scale factor error.The present invention is intended to reduce to a minimum the chances ofthe need for mode shifting occurring during actual operation of thegyroscope in some changing environment.

The above features of the present invention have been given in a generalway in order that the more detailed description of the invention whichfollows may be better understood and the contribution to the art betterappreciated.

The full scope and content of the present invention will be appreciatedby referring to the detailed description of the preferred embodiment inconjunction with the appended drawings wherein like reference numeralshave been used to refer to like elements, and in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a cavity length control circuitemploying digital data processing;

FIG. 2 shows a graphical representation of a plurality of opticalintensity maxima at different startup temperatures, with and withoutintermediate secondary peaks;

FIG. 3 shows a portion of the computer program flow diagram for digitaldata processing; for example, in a microprocessor to control the ringlaser gyroscope; and

FIGS. 4, 4A and 4B show in more detail the CLCSWEEP portion of theprogram shown in FIG. 3.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Turning now to FIG. 1, a cavity length control circuit 10, according tothe present invention provides a means by which the optical cavity pathlength for the ring laser gyro may be tuned to a selected intensitymaximum. For example, for a 28 centimeter ring laser gyro, cavitydetuning may be given in frequency units of MHz, where the equivalentdetuning frequency for one longitudinal mode change is 1,071 MHz.

As is shown in FIG. 2, the amplitude of the sum of the intensities ofthe counter-rotating beams passes through a number of maxima as thevoltage applied to the piezoelectric transducer varies about a zerovoltage point to increase the optical cavity length by increasing thevoltage from some minus value to some plus value. The distance L betweenthe peaks, for a 28 centimeter ring laser gyro cavity, mentioned above,is the equivalent of one longitudinal mode change. Intervening secondarypeaks may occur between the maxima, as is also shown by the solid anddashed curves in FIG. 2. It will be understood by those in the art thatthe physical parameters of the ring laser gyro may result in more thanfour maxima peaks within the cavity path length mirror sweep and alsomay result in the absence of secondary peaks, (dot-dash curve in FIG.2). The cavity length control circuit 10 according to the presentinvention maintains the cavity length such that the optical intensitystays at a selected maxima, i.e., corresponding to a selected integermultiple of the wavelength of the ring laser gyro beams. Thus, the modecenter or intensity maximum represents a fixed optical frequency of thering laser gyro determined by the parameters of the lasing medium.Variations in the cavity length about the mode center cause variationsin the resonance frequency of the cavity and intensity variations due tothe frequency dependency of the gain of the ring laser gyro.

The cavity length control circuit 10 provides an input to the ring lasergyro 12, which may be, for example, an LG8028B ring laser gyro,manufactured by the assignee of the present invention. A similargyroscope, except with a 17 cm cavity, is discussed in Grant, Jr., "TheLitton LC-2717 Laser Gyro", NAECON Proceedings, June, 1979, thedisclosure of which is hereby incorporated by reference. This inputsignal provides a driving signal to a pair of piezoelectric transducers14 and 16 which drive the mirrors associated with the piezoelectrictransducers 14 and 16 to maintain a detune of zero, i.e., maintain theoptical intensity, as sensed by the photo-detector behind a third of thering laser gyro mirrors, at an optical maxima. The output of thephoto-detector measuring the optical intensity is an input to a detunefeedback loop through an amplifier 18, having the intensity connected toits positive terminal and its output fed back to its negative terminalthrough feedback loop 20.

The amplifier 18 and feedback loop 20 together form essentially a highpass filter having about a one second time constant, i.e., about 1.5 Hz(1 radian/sec/ 2π). The output of the amplifier 18 is the A.C. componentof the optical intensity; due to the dithering of the mirrors at somefrequency, e.g., 5050 Hz. The output of the feedback loop 20 is the D.C.value of optical intensity. The output of the amplifier 18 is A.C.intensity input to the gyro electronics portion 22 of the cavity lengthcontrol circuit 10. This input is fed through a bandpass filter 24,having a center frequency of a selected value, for example, 5,050 Hz(nominally 5k Hz). The output of the bandpass filter 24 is demodulatedin a phase sensitive synchronized demodulator 26 which demodulates witha reference oscillation at the 5050 Hz reference from a referenceoscillator 25. The output of the demodulator 26 is further filteredthrough a lowpass filter 28 having a cut-off frequency of approximately4 Hz. The output of the lowpass filter 28 forms an output of the gyroelectronics circuit 22 and an input to the digital data processorassociated with the ring laser gyro through an analog-to-digitalconverter 30. The digital output of the analogue-to-digital converter30, which samples the analogue signal output of the filter 28 at a rateof 215 Hz forms an input CLCMONlX to the I/O processor 32 which isrepresentative of the feedback detune value. The feedback detune valueCLCMONlX is essentially the slope of the A.C. optical intensity at anygiven cavity path length control position of the mirrors. Therefore,when CLCMONlX is equal to zero, either a peak or a valley exists. Thefeedback detune value CLCMONlX is filtered by a digital filtercharacterized by a function G(z) in the I/O processor 32, as is known inthe art for noise reduction, to produce a filtered error value FLTRERRX.

The FLTRERRX signal is then digitally integrated by an integrator 35 inthe I/O processor 32. The function KΣ is the programmed trackingfunction when the gyroscope is in the tracking mode. The value k is asoftware gain factor set, e.g., at 1/8 or 1/32 for, respectively acapture mode where more radical changes in mirror position are needed toobtain a detune of O and at 1/32 for normal tracking to maintain adetune of O.

The output of the integration of the signal FLTRERRX is a digital signalCLCOUTX, the output control value for the transducers 14 and 16, whichforms an input back into the gyro electront 22 through adigital-to-analog converter 34, which also includes a sample and holdcircuit. The output of the digital-to-analogue converter 34 is modulatedwith the 5050 Hz reference oscillation and amplified in an amplifier 36to provide the control signal to the piezoelectric transducers 14 and 16in the ring laser gyroscope 12.

A timing sequence at a time shortly after turn-on for a ring laser gyro,according to present invention, is given in TABLE 1.

                  TABLE 1                                                         ______________________________________                                        TIMING SEQUENCE AT TURN-ON                                                    TIME (SEC)      DESCRIPTION                                                   ______________________________________                                        0               Start CLC Sweep                                               0-5             Map CLC and Log all                                                           Intensity Peaks                                               6               Select a Peak as a                                                            Function of Startup                                                           Temperature                                                   7               Set CLC to Selected Peak                                                      Engage CLC Track Mode                                         ______________________________________                                    

The timing sequence assumes, e.g., the presense of three ring lasergyros, having their axes of rotation orthogonal to each other, mountedon a common block. During the period of time in startup before somearbitrarily selected time 0 in Table 1, the gyro, under programmedcontrol, is establishing certain operating parameters and startupfunctions, e.g., dither track control is engaged, the gyros are swept toprovide data for the amplitude control loop of the dither control anddrive of all gyros is engaged. Along with the engaging of the dithertrack control, high voltage enable is set and ignition aid is turned on.At time 0 in Table 1 the CLC mode selection is begun. A sweep of the CLCmirrors is conducted, e.g., starting at the high voltage end or lowvoltage end without the tracking function kΣ engaged. The mirrors arethen driven by the circuit 10 of FIG. 1 through a range of operation,e.g., full scale from the maximum to the minimum operating voltage tothe piezoelectric transducers 14 and 16. The processor 32 maps theintensity values and locates and stores the positions of all intensitypeaks occurring during the sweep, during, e.g., time=0 sec. to time=6sec. Thereafter, based upon startup temperature, a peak is selected asthe gyro mode and the mirrors are driven to set the cavity length toresonate at the mode peak selected. Tracking is engaged and thereaftermaintains the intensity at the selected mode peak as described above. Itwill be seen from the computer flow diagram of FIG. 3 that the cavitylength control loop CLCNTL is not engaged until after high voltage ondue to the determination being made in Block 100 of the program ofwhether high voltage is on or not. If high voltage is not on, the cavitylength control loop goes to return in Block 102. If high voltage is on,as shown in Table 1, a determination is made in Block 104 whether theignition aid has been turned on. If ignition aid is on, the temperatureof the ring laser gyro is determined in Block 106. Also, in Block 106,the cavity length control voltage is ramped to its positive or negativeextremity in Block 106. The decision on which end is selected is basedupon startup temperature. It may be possible, therefore, to sweep only aportion of the CLC mirror travel. This is due to the fact that theinitial mode peak selected will also be temperature dependent. Thus,e.g., if the initial mode peak to be selected is one of those to theright of 0 volts in FIG. 2, the cavity need only be swept from themaximum voltage down to 0 volts to map and locate the appropriate peak.After ignition aid has been turned on, as determined by the no decisionin Block 104, it is determined in Block 108 whether the CLC track modehas been engaged. Prior to the engagement of the CLC track mode, thecavity length control acquire logic performs the CLCSWEEP loop explainedin more detail in connection with FIGS. 4, 4A and 4B. In this loop, theoptical intensity peaks are sensed and mapped through the entire span ofthe slewing capability of the cavity length control circuitpiezoelectric controlled mirrors or some selected portion thereof, andthe primary maxima locations determined and stored in memory. Based uponthe initial temperature of the ring laser gyro, and according to logiccontained in the cavity length control program in the microprocessor, aninitial one of the maxima is selected for a mode about which the controlcircuit 10, referred to above, maintains zero detune.

When in the track mode, the program shown in FIG. 3 executes the programto obtain the signal CLCOUTX output of the I/O processor 32. In Box 110,a value Δ0 is set to equal minus the error signal CLCMONlX minus theprevious value for FLTRERRX. A new value for FLTRERRX is set to equalthe previous value of FLTRERRX plus Δ divided by four. A determinationis then made in Block 112 whether the absolute value OF FLTRERRX isgreater than some selected value. If so, the software gain factor is setto 1/8, indicating the capture mode which provides a more rapid changein the error signal. This occurs when the value of FLTRERRX indicatesthat the feedback error signal is larger than some selected value,indicating the cavity path length differs from the desired maxima pointsufficiently, (i.e., detune differs sufficiently from 0), to requiremore radical changes in the control signal to more rapidly move thecavity length control circuit piezoelectrically controlled mirrors, toreattain the desired intensity maximum. If the decision in Block 112 isa no, the software gain control factor, k is correspondingly set to asmall value 1/32. The output CLCOUTX of the I/O processor 32 is then setto the previous value of CLCOUTX plus k times the value of FLTRERRX.

Turning now to FIG. 4, the program by which the microprocessordetermines the location of the available maxima in the sweep of thecavity length control mirrors and selects a desired initial maxima pointto obtain a selected integer multiple of the laser wave length, and alsoaccommodate the predicted temperature range of operation, in order toavoid the necessity for mode hop during operation, is shown in furtherdetail.

FIG. 4 shows a flow diagram for a program for the microprocessor 32 toinitialize CLC starting in Block 120. In Block 122 the startuptemperature value is determined. Signals indicative of temperature inputto the microprocessor 32 may be provided from a sensor on the ring laserbody, as shown, e.g., in U.S. Pat. No. 4,314,174 to Wing et al, assignedto the assignee of the present invention, the disclosure of which ishereby incorporated by reference. In Block 124 a determination is madewhether the temperature is greater than or equal to some selected value,e.g., 20° C. If so therefor, the program executes the command in Block126 to set CLC to the plus end. In this way the mirrors are driven withthe high end of the piezoelectric transducer control voltage so thatmapping commences from the largest cavity path length given the startuptemperature. If the temperature at startup is determined to be less than20° C., then a determination is made in Block 128 whether thetemperature at startup is less than 0° C. If the temperature is greaterthan or equal to 0° C., the program executes the command in Box 130 toset the voltage at the mid-point, i.e., 0 volts, to set the mirrors fora mid-point cavity length. If the temperature is less than 0° C., theprogram executes the command in Box 132 to set the voltage to thetransducers at the maximum minus value to place the mirrors at the minusvoltage end of the CLC sweep, so that the mapping commences from theminimum cavity path length position of the mirrors.

Once initialization is complete, the program moves from Box 134 to Box150 where the sweep position of the mirrors is incremented by changingthe control voltage by some small value δ, e.g., 0.2 volts. The signalFLTRERRX (the digitally filtered error signal) representing the slope ofthe A.C. intensity curve, is then compared to 0 in Block 152. If it isnot zero, the determination is made in Block 154 whether the sweep hasbeen completed. This, as explained below, may be the completion of asweep from the high voltage of e.g., +9 volts to the low voltage of,e.g., -9 volts or, e.g., from 0 volts to +9 volts and is preferably alsosoftware controlled.

If the determination is made in Box 152 that the CLC error is equal tozero, either a peak or valley exists at that value of the piezoelectriccontrol voltage. In order to determine whether a peak or valley exists,and also to eliminate secondary peaks, a determination is made in Block156, whether the optical intensity is greater than some minimumthreshhold value, e.g., 80% of the empirically determined peak value fora given gyroscope. If the value is greater than the selected threshhold,the command is executed in Block 158 to log the CLC position. That is,the control signal voltage to the piezoelectric transducers is stored ina selected memory location. If such a value is stored or if theintensity does not exceed the threshhold, the sweep voltage isincremented, unless the sweep is complete, as a result of thedeterm.ination made in Box 154.

Once the sweep is complete, several intensity maxima will be stored,each in a respective memory location. For example, if the sweepcommenced from the maximum minus voltage and the gyro exhibits fourmaximum peaks over the sweep range to the maximum plus voltage, the fourpeaks will be stored sequentially, i.e., from left to right as shown inFIG. 2, in four selected memory locations. The program then executes theportion which selects a particular peak dependent upon gyro startuptemperature. The listed proqram (FIG. 4B) assumes a gyro which exhibitsfive peaks over the CLC sweep range, with one centered at about 0 voltsto the transducers. Also, assumed for discussion here is that the cavitylength control mirror sweep was from the maximum negative voltage end ofthe sweep to the maximum positive voltage end and the peak locationswere stored, sequentially, in five selected memory locations.

In Block 160 a determination is made whether the startup temperature isless than -20° C. If it is less than -20° C., the command is executed inBlock 162 to set the cavity length control mirrors to the maximumnegative location of a peak determined during the sweep. That is, thefirst of the sequentially stored maxima locations is selected frommemory. If the temperature is equal to or exceeds -20° C., a decision ismade in Block 164 whether the temperature is less than 0° C. If so, thenext sequentially stored maxima is selected from memory according to thecommand in Block 166. Next, the program determines in Block 168 whetherthe startup temperature is less than 20° C., and if so, the nextsequentially stored maximum location is selected from memory in Block170, which would be the one centered closest to 0 volts. In Block 172 adetermination is made, if the decision in Block 168 is that startuptemperature equals or exceeds 20° C., whether the startup temperature isless than 40° C. If it is, the next sequentially stored maximum locationis read from memory; and if temperature equals or exceeds 40° C., thepeak corresponding to the highest plus voltage is selected from memory.

Once a peak is selected in the manner prescribed above, CLCSWEEP iscomplete and the program initiates the tracking function describedabove, wherein the integral error signal kΣ is employed to maintain theselected mode peak by keeping detune at 0.

FIG. 4B shows a program for a specific embodiment of the presentinvention employed in an 8028 ring laser gyro manufactured by theassignee of the present invention. It may be for some ring laser gyros,as it is for the 8028, that only a few (less than four at most) peaksexist in the sweep range of the CLC mirrors, and secondary peaks areabsent or of negligible significance. In this case, it may empiricallybe determined that, at a selected voltage to the transducers, it isreasonably certain that the mirrors will be positioned to result in acavity length on the upslope of the intensity curve at the desired oneof the maxima, regardless of startup temperature.

In FIG. 2, (dot-dash curve), the intensity curve for the 8028 is drawnroughly to scale to illustrate that there are only three maxima and nosecondary peaks. It will also be seen that selecting -4.5 volts, 0 voltsor +4.5 volts will be adequate for three temperature bands, e.g.,<10°C.; ≧10° C., ≦40° C.; and >40° C. to select the appropriate one of themaxima.

Two factors contribute to the requirement for mapping and maxima storageexplained above when the number of maxima increase, and/or secondarypeaks occur. Due to startup temperature variation, the curve shifts dueto temperature induced cavity length charges, as shown in FIG. 2, forexample, for 20° C. and 70° C. startup temperature curves. Due to this,it is more difficult to select fixed voltages which will insure arrivingat the proper peak. Also, if the fixed voltage should happen to be at atrough, it cannot be predicted to which of the maxima bracketing thetrough the tracking function will track. The presence of secondary peaksincreases the number of troughs, and thus, the uncertainty factor iffixed voltages are used. However, it has been found that with the 8028the simple expedient of selecting one of three piezoelectric drivevoltages, based upon startup temperature, will result in the properintensity peak being selected as the gyro mode about which detunetracking is then carried out.

The program shown in FIG. 4B determines in Block 200 whether the startuptemperature is less than, e.g., 40° C. If it is not, then thepiezoelectric control voltage is set to +4.5 volts in Block 202, and thetracking function is engaged. As seen in FIG. 2, this results inpositioning the CLC mirrors such that the intensity is on the upslope tothe rightmost peak. Tracking then sets the cavity path length to attainthis mode peak and maintain this mode.

If the temperature is determined in Block 200 to be less than or equalto 40° C., a determination is made in Block 204 whether the temperatureis less than 0° C. If it is not, the CLC mirror drive voltage is set to0 volts in Block 206 and if it is, the CLC mirror drive voltage is setto -4.5 volts in Block 208. In this manner, as described above,tracking, once engaged, will set the cavity length to, respectively, themode just to the right of 0 volts or the mode to the left of 0 volts. Ithas been found that, even with a shift of the maxima curve, due tostartup temperature variation, these voltages will still lie on thedesired maxima upslope and enough away from the trough between theadjacent maxima to insure the proper mode is reached.

It will be seen by those skilled in the art that the present inventionprovides significant improvements to cavity path length control in ringlaser gyros. Cavity path length is selected at startup, based uponstartup temperature, to obtain a length which is the same integermultiple of the laser wavelength from startup to startup, regardless oftemperature variations. In addition, the selected position of themirrors gives the greatest degree of allowable cavity path lengthcontrol mirror movement for expected temperature variations from thetemperature at startup. The system is flexible enough to accommodate acontrol at startup, based upon empirically determined laser cavitycharacteristics for some lasers, in which startup temperature dictateswhich of a plurality of predetermined voltage is to be applied to thepiezoelectric transducers of the cavity path length control mirrors. Thecontrol voltage is also based upon actual mapping of the intensityvariation due to changing the mirror positions, (i.e., the cavity pathlength), for those lasers whose intensity variation characteristics areless susceptible to the fixed selected voltage manner of control. Inthis latter manner of control, the mapped intensity peaks are stored,and one is selected for the cavity path length control mode, based uponstartup temperature.

Those skilled in the art will appreciate that many changes andmodifications to the present invention may be made without departingfrom the scope and intent of the present invention. For example, thepresent invention has been described as being digitally implemented in amicroprocessor programmed to carry out the present invention. It will beunderstood that, for example, large scale integrated circuits could bespecifically constructed to perform the programmed functions. Also, theprogrammed functions, depending upon laser cavity characteristics couldemploy a greater or lesser number of temperature ranges upon which tobase the selection of the startup mode. Also, the peak determination maybe made and detune controlled using the intensity rather than the slopeof the intensity.

These and other modifications will be apparent to those skilled in theart. It is the intention of the applicants, in the appended claims, tocover all such changes and modifications as come within the scope andcontent of the present invention.

What is claimed is:
 1. In a ring laser gyroscope having a plurality ofmirrors defining a path length and means for moving a selected one ofthe mirrors to change the cavity path length, a cavity path lengthcontroller, comprising:means for sensing the startup temperature of thering laser gyroscope; means for dividing the difference between amaximum operational temperature and a minimum operational temperature ofthe ring laser gyroscope into a plurality of temperature ranges;temperature range determining means for determining in which of theplurality of temperature ranges the startup temperature is; and cavitylength initializing means for positioning the selected cavity pathlength control mirror to a selected position based upon thedetermination made by the temperature range determining means.
 2. Thering laser gyroscope of claim 1 wherein cavity length initializing meansfurther comprises:control means for controlling the means for moving theselected mirror through its entire sweep of cavity path length controlmovement at the startup of the ring laser gyroscope; means for sensingthe occurrence and locations of all intensity maxima in the opticalintensity of the sum of the counterpropagating beams during the movementof the mirrors through the entire sweep; and means for operating thecavity length initializing means to move the selected mirror to amaximum intensity location selected as a function of the temperature ofthe ring laser gyroscope at startup.
 3. The ring laser gyroscope ofclaim 2, wherein the control means for controlling the means for movingthe selected mirror comprises means for generating a control voltagevarying over a selected voltage range; and the means for operating themeans for moving the selected mirror to a selected maximum intensitylocation comprises means for storing a value of the control voltagedetermined during the sweep of the mirror and applying that voltage tothe means for moving the selected mirror.
 4. The ring laser gyroscope ofclaim 2, wherein the control means for moving the selected mirrorthrough the entire sweep comprises means for generating a controlvoltage in stepwise increments through a selected voltage range; and themeans for operating the means for moving the selected mirror to aselected intensity maximum comprises means for storing digital signalsindicative of the particular step in the stepwise incrementation of theselected mirror through the sweep where each of the sensed maximaoccurs, and means for recovering from memory a selected digital datasignal in response to the startup temperature.
 5. The ring lasergyroscope of claim 4, further comprising threshold determination meansfor determining whether the intensity at each of the sensed maximumpoints exceeds a selected threshold intensity.
 6. The apparatus of claim1 wherein the cavity length initializing means further comprises:meansfor comparing the temperature at startup with a first temperature andfor generating a first control signal if the temperature at startup isgreater than the first temperature and, if not, for initiating a secondcomparison; means for comparing the temperature at startup with a secondtemperature and for generating a second control signal if thetemperature at startup is greater than the second temperature, and, ifnot, for initiating a third comparison; means for comparing thetemperature at startup with a third temperature and for generating athird control signal, if the temperature at startup exceeds the thirdtemperature, and for generating a fourth control signal, if thetemperature at startup does not exceed the third temperature.
 7. Theapparatus of claim 6 further comprising:data storage means for storingin separate storage locations, data representative of a plurality ofselected cavity length mirror positions; means for addressing the datastorage means at a particular one of the storage locations in responseto one of the first, second, third or fourth control signals.
 8. Theapparatus of claim 7 further comprising:means for loading the outputs ofthe means for sensing the occurrence of an intensity maximum and thelocation of the maximum during the sweep of each of a plurality of suchintensity maxima into a respective one of the separate storagelocations.
 9. The apparatus of claim 1 wherein the cavity lengthinitializing means further comprises:means for providing one of aplurality of distinct predetermined output signals unique to each of theplurality of temperature ranges in response to the determination by thetemperature range determining means of the existence of a startuptemperature within one of the temperature ranges.
 10. The apparatus ofclaim 9 further comprising:means for providing a drive voltage to themeans for moving the at least one of the mirrors, having a range ofpossible operating voltages from a first negative value to a firstpositive value; the plurality of temperature ranges is three, includinga first range of less than a first temperaature, a second range ofgreater than or equal to the first temperature and less than or equal toa second temperature, and a third range of greater than the secondtemperature; a first control signal associated with the firsttemperature range is a negative voltage of about one-half the firstnegative voltage, and a second control signal associated with the secondtemperature range, is about zero volts, and a third control associatedwith the third temperature range is about one-half of the first positivevoltage.
 11. The apparatus of claim 10, further comprising:means forcontrolling the means for moving the mirrors to position the mirrors atthe intensity maximum closest to the position of the mirrors resultingfrom the one of the first, second or third control signals generatedresponsive to the range in which the startup temperature is.
 12. Acavity length controller for a ring laser gyroscope having a pluralityof mirrors that define a cavity length and including means for moving atleast a selected one of the mirrors to change the cavity length,comprising:means for sensing the startup temperature of the ring lasergyroscope; control means for sweeping the selected mirror through itsentire range of cavity length control movement at the startup of thering laser gyroscope; means for sensing the occurrence and locations ofall intensity maxima in the sum of the counterpropagating beams duringthe movement of the selected mirror through the entire sweep; and meansfor moving the selected mirror to a mazimum intensity location selectedas a function of the temperature of the ring laser gyroscope at startupto provide a predetermined cavity length.
 13. A cavity length controllerfor a ring laser gyroscope having a plurality of mirrors that define acavity length and including means for moving at least a selected one ofthe mirrors to change the cavity length, comprising:sensing the startuptemperature of the ring laser gyroscope; initializing means for movingthe selected mirror to a location selected as a function of the startuptemperature to provide a predetermined scale factor for the ring lasergyroscope at startup; control means for sweeping the selected mirrorthrough its entire range of cavity length control movement at eachstartup of the ring laser gyroscope; means for sensing all intensitymaxima in the sum of the counterpropagating beams during the movement ofthe selected mirror through the entire sweep; and means for positioningthe selected mirror at a location to produce an intensity maximumcorresponding to a predetermined cavity length.